The present invention relates generally to silicon carbide and, more particularly, to a method and apparatus for growing low defect density silicon carbide.
Silicon carbide (SiC) has a number of characteristics that make it an ideal candidate for a variety of semiconductor applications, primarily those requiring high power handling capabilities. Arguably the most important characteristic of SiC is its indirect bandgap, resulting in relatively high recombination lifetimes and the ability to produce higher voltage junctions than those that can be produced from a direct bandgap material. The large bandgap of this material also provides for negligible current leakage up to 500xc2x0 C., thereby allowing for high temperature operation without excessive leakage current or thermal runaway. The switching frequency of SiC devices is much higher than that of a device fabricated from silicon or gallium arsenide due to SiC""s high breakdown strength and the resultant reduction in minority carrier storage and associated switching losses. Lastly, due to the high junction temperature and the high thermal conductivity of SiC, devices fabricated from SiC have reduced cooling requirements.
Although semiconductor devices based on SiC offer vast improvements over devices fabricated from silicon, in order to realize these improvements materials must be fabricated with much lower defect densities than have been obtainable heretofore. As noted by the authors in the 1999 article entitled SiC Power Devices, Naval Research Reviews, Vol. 51, No. 1(1999), in order to scale up devices fabricated from SiC, the density of dislocations as well as the density of micropipes must be reduced. Conventional SiC material has a dislocation density between 105 and 106 per square centimeter and a micropipe density between 102 and 103 per square centimeter. Some extremely high quality SiC material has been grown with dislocation densities on the order of 104 per square centimeter. Unfortunately, even this dislocation density is at least an order of magnitude too high for many semiconductor applications. Id. at page 21.
U.S. Pat. No. 5,679,153 discloses a technique of growing SiC epitaxial layers using liquid phase epitaxy in which the density of micropipes is substantially reduced or eliminated. In one aspect of the disclosed technique, an epitaxial layer of SiC is formed on a bulk single crystal of SiC, the epitaxial layer being of sufficient thickness to close micropipe defects propagated from the bulk crystal. In order to form an electronically active region for device formation, a second epitaxial layer is formed on the first epitaxial layer by chemical vapor deposition. Based on this technique, SiC layers having micropipe densities of between 0 and 50 micropipes per square centimeter on the surface were claimed.
Although techniques have been disclosed to achieve SiC materials with low micropipe densities, these techniques do not lend themselves to growing bulk materials, i.e., materials that are at least a millimeter thick or more preferably, at least a centimeter thick. Additionally, these techniques do not impact the dislocation densities of the material. Accordingly, what is needed in the art is a technique of growing bulk SiC material with defect densities on the order of 103 per square centimeter, more preferably on the order of 102 per square centimeter, and even more preferably on the order of 10 or less dislocations per square centimeter. The present invention provides such a technique and the resultant material.
In accordance with the invention, a low defect density silicon carbide (SiC) is provided as well as an apparatus and method for growing the same. The SiC crystal, grown using sublimation techniques, is divided into two stages of growth. During the first stage of growth, the crystal grows in a normal direction while simultaneously expanding laterally. Preferably during this stage the ratio of the lateral growth rate to the axial growth rate is between 0.35 and 1.75. Although dislocations and other material defects may propagate within the axially grown material, defect propagation and generation in the laterally grown material are substantially reduced, if not altogether eliminated. After the crystal has expanded to the desired diameter, the second stage of growth begins in which lateral growth is suppressed and normal growth is enhanced. Preferably during this stage the ratio of the lateral growth rate to the axial growth rate is between 0.01 and 0.3, and more preferably between 0.1 and 0.3. A substantially reduced defect density is maintained within the axially grown material that is based on the laterally grown first stage material. Preferably during this stage the ratio of the lateral growth rate to the axial growth rate is between 0.01 and 0.3, and more preferably between 0.1 and 0.3. A substantially reduced defect density is maintained within the axially grown material that is based on the laterally grown first stage material.
In one aspect of the invention, a SiC material is provided with a low defect density, defects including both dislocations and micropipes. The defect density in the grown SiC is less than 104 per square centimeter, preferably less than 103 per square centimeter, more preferably less than 102 per square centimeter, and still more preferably less than 10 per square centimeter. In at least one embodiment, SiC is grown comprised of an axially grown region and a laterally grown region, the laterally grown region having the desired low defect density. In another embodiment of the invention, the SiC is comprised of a central region having a first defect density and a perimeter region encircling the central region that has a second defect density. The first defect density may be greater than 104 per square centimeter. The second defect density is substantially less than the first defect density and is less than 103 per square centimeter, preferably less than 102 per square centimeter, and more preferably less than 10 per square centimeter. In another embodiment of the invention, the SiC material is comprised of a SiC seed crystal, a first crystalline growth region initiating at a growth surface of the SiC seed crystal and following an axial growth path, and a second crystalline growth region of the desired defect density initiating at a growth surface of the SiC seed crystal and following a laterally expanding growth path. The defect density in the first crystalline growth region may be greater than 105 per square centimeter. The laterally expanding growth path is at an angle of at least 25 degrees, and preferably at least 45 degrees, from the normal, i.e., axial, growth path.
In another aspect of the invention, a method of growing a SiC material with a low dislocation density is provided. In at least one embodiment, a SiC seed crystal is introduced into a sublimation system wherein both axial and lateral crystal growth is promoted, at least during one stage of growth. Propagation of dislocation defects, including micropipes, from the seed crystal into the laterally grown crystal is substantially reduced as is generation of dislocation defects within this region. In at least another embodiment of the invention, a SiC seed crystal is introduced into a sublimation system and heated to a temperature sufficient to cause sublimation. Temperature gradients within the sublimation system as well as temperature differentials between the crystallization growth front and adjacent surfaces promote a first stage of free space crystal expansion wherein the crystallization front expands both axially and laterally followed by a second stage of free space crystal expansion wherein the crystallization front expands axially while lateral expansion is suppressed.
In another aspect of the invention, an apparatus for use in growing a SiC material with a low dislocation density is provided. In at least one embodiment of the invention, the apparatus includes a ring element that promotes lateral crystal expansion, preferably through the use of a conical surface. The ring element may also be used to shield the edge of the SiC seed from the growth process. The ring element may also include a second surface, preferably conical, that promotes lateral crystal contraction. Preferably the ring element inner surfaces are comprised of either TaxCy or NbxCy. In at least one embodiment of the invention, the apparatus also includes a graphite heat sink coupled to a non-growth surface of the SiC seed crystal, a growth chamber with inner surfaces preferably comprised of either TaxCy or NbxCy, and means for applying temperature gradients to the crucible.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.